The present invention relates to a process for the preparation of .alpha., .alpha., .alpha., .alpha.', .alpha.'-pentachloro-o-xylene by the photochlorination of o-xylene.
In o-xylene, only 5 of the 6 hydrogens of the side chain can be replaced by Cl. The literature on the preparation of o-pentachloroxylene is not very plentiful, and is mostly contained in the literature on the chlorination of m-xylene and p-xylene. The methods of preparation are the same as the method of radical chlorination. Colson and Gouthier (A.Ch. (6) 11, 26) chlorinated with PCl.sub.5. Light, or radical forming catalysts are used as radical sources (A. E. Kretow et al., Z. Abscei Chimii 30, 3019-24 (1960), German Reich Patent 668,033, DL Patent 9,096, E. T. McBee et al. Natl. Nuclear Energy Ser. Div. VII, 1,207-21 (1951), Harvey et al, J. Appl. Chem., 4, 319-25 (1954). Data concerning yield and purity are lacking in most of the literature. According to McBee et al., a product forms in the chlorination of o-xylene which contains 67.1% chlorine, but the theoretical chlorine content of o-pentachloroxylene is 63.7%. Only by distillation is a product considered as pure obtained with 64.1 % chlorine plus higher-boiling fractions containing 67.5 and 78.2% chlorine. But distillation residues, which certainly are produced, are not mentioned, but decompositions towards the end of the distillation are mentioned.
Excessively high chlorine contents are to be attributed to chlorination in the nucleus and chlorolysis. Chlorolysis leads primarily to products of low chlorine content. A. E. Kretow finds, in addition to the main product, an oil having a lower chlorine content of 51%.
Since in the chlorination of o-xylene one must expect unexplained secondary reactions in addition to the known reactions, the preparation of a pure product is difficult.